Pentanals, also called valeraldehydes, have gained economic significance as intermediates in industrial organic chemistry. They occur in four different structural isomers as linear n-pentanal, branched 2-methylbutanal, branched 3-methylbutanal and highly branched 2,2-dimethylpropanal or pivalaldehyde. Pentanals can be used as such, for example for the preparation of fragrances, or in the form of the derivatization products thereof, such as pentanols, pentanoic acids or pentylamines. The pentanals can be used in pure isomeric form or in the form of an isomer mixture (Weissermel, Arpe, Industrielle Organische Chemie [Industrial Organic Chemistry], 3rd edition, VCH Verlagsgesellschaft mbH, Weinheim, 1988, page 218; Schneidmeir, Chemiker-Zeitung, volume 96 (1972), no. 7, pages 383-387).
Because of the reactive aldehyde group, pentanals in a basic medium can enter into an aldol addition reaction to give pentanal dimerization products having ten carbon atoms. If at least one of the pentanal isomers has two reactive hydrogen atoms in the α position to the carbonyl group, the aldol addition product formed at first can be converted to α,β-unsaturated decenal with elimination of water. For example, the self-addition of n-pentanal with subsequent elimination of water forms 2-propylheptenal, which can be converted by complete hydrogenation to 2-propylheptanol, which finds use as a plasticizer alcohol (EP 0 366 089 A2). The selective hydrogenation of 2-propylheptenal at first gives 2-propylheptanal, which can be converted by subsequent oxidation to 2-propylheptanoic acid. 2-Propyl-heptanoic acid can then be used as acid component for preparation of lubricant esters.
Pentanals are prepared industrially by reaction of butenes with synthesis gas, a mixture of carbon monoxide and hydrogen, in the presence of transition metal compounds. The reaction of olefins with synthesis gas is also referred to as the hydroformylation reaction or oxo reaction, and the hydroformylation of but-1-ene affords, as well as the straight-chain n-aldehyde n-pentanal, also certain proportions of the isoaldehyde 2-methylbutanal (Ullmann's Encyclopedia of Industrial Chemistry, 6th edition, 2003, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, vol. 2, pages 73-74; vol. 25, pages 286-289).
Butenes are obtained industrially by the steamcracking of naphtha. Typically, separation of 1,3-butadiene from the butene cut from the naphtha cracking to form raffinate I is followed by separation of isobutene to form raffinate II (Weissermel, Arpe, Industrielle Organische Chemie, 3rd edition, VCH Verlagsgesellschaft mbH, Weinheim, 1988, pages 71-79). For the subsequent hydroformylation reaction, predominantly raffinate II is used, in which a small residual isobutene content can be permitted. In special cases, it is also possible to process raffinate I having a high isobutene content, and occasionally also a but-1-ene-depleted raffinate II, which can also be referred to as raffinate III. The hydroformylation reaction can be conducted either in the presence or in the absence of complex-forming compounds, for example in the presence of organophosphorus compounds. According to EP 0 366 089 A2, a homogeneous organic solution is employed with rhodium triphenylphosphine catalysis. Since a maximum proportion of n-pentanal compared to 2-methylbutanal in the pentanal mixture formed is generally the aim, the hydroformylation reaction is frequently conducted in the presence of homogeneously dissolved transition metal complexes, which first enable isomerization of the but-2-ene to but-1-ene, which is then hydroformylated predominantly to n-pentanal. Rhodium complex catalysts suitable for the isomerizing hydroformylation of a mixture of linear butenes are described, for example, in DE 102 25 282 A1, in which the complex ligands have a xanthene skeleton.
Rhodium complex catalysts based on bisphosphite ligands together with sterically hindered secondary amines, which are likewise suitable for the isomerizing hydroformylation of a mixture of linear butenes, are discussed in DE 10 2008 002 187 A1. Two-stage process variants are also known, for example according to DE 43 33 324 A1, DE 42 10 026 A1, DE 101 08 474 A1 and DE 101 08 475. In the first stage, preferably but-1-ene is converted, while, in the second stage, the but-2-ene-containing offgas from the first stage is hydroformylated to give a mixture of n-pentanal and 2-methylbutanal. According to DE 43 33 324 A1 and DE 101 08 474 A1, the first hydroformylation stage can also be conducted in the presence of water-soluble rhodium complex catalysts. In this type of reaction regime, a liquid aqueous catalyst solution is present alongside the liquid organic reaction solution, which, after leaving the hydroformylation zone, can be separated from one another in a simple manner by phase separation. Because of the presence of an aqueous phase and an organic liquid phase, this type of reaction regime is also referred to as a heterogeneous process or biphasic process.
According to the composition of the butene feed mixture and the reaction conditions in the hydroformylation stage, a pentanal mixture is obtained with varying proportions of n-pentanal, 2-methylbutanal, 3-methylbutanal and a small amount of pivalaldehyde, which is typically distilled. n-Pentanal has a boiling point of 103° C. at standard pressure and can be removed with the higher boiler stream. Because of the small difference in boiling point between 2-methylbutanal (92° C. at standard pressure) and 3-methylbutanal (92.5° C. at standard pressure), 3-methylbutanal cannot be separated completely from the 2-methylbutanal with acceptable distillation complexity. If the intention is to obtain n-pentanal of maximum purity, the distillation step is frequently laid in such a way that the more volatile 2-methylbutanal, as well as the content of 3-methyl-butanal, still contains considerable amounts of n-pentanal which are removed with the volatile constituents. There is therefore a need for a method of utilizing the amounts of n-pentanal still present in the 2-methylbutanal removed and the residual contents of 3-methylbutanal present with maximum economic viability. At the same time, 2-methylbutanal is to be obtained with a residual 3-methylbutanal content of less than 0.2% by weight, based on the organic component, which is in demand for particular applications, for example for fragrance production.